Oligo- or polyamines as oxidation stabilizers for biofuel oils

ABSTRACT

The use of oligo- or polyamines which have a number-average molecular weight of from 46 to 70 000 and are free of phenolic hydroxyl groups for increasing the oxidation stability of biofuel oils based on fatty acid esters, or of mixtures of such biofuel oils with middle distillates of fossil origin and/or of vegetable and/or animal origin, which are essentially hydrocarbon mixtures and are free of fatty acid esters.

The present invention relates to the use of oligo- or polyamines which have a number-average molecular weight of from 46 to 70 000 and are free of phenolic hydroxyl groups for increasing the oxidation stability of biofuel oils based on fatty acid esters, or of mixtures of such biofuel oils with middle distillates of fossil origin and/or of vegetable and/or animal origin, which are essentially hydrocarbon mixtures and are free of fatty acid esters.

The present invention further relates to a mixture of specific representatives of such oligo- or polyamines and biofuel oils based on fatty acid esters.

The present invention further relates to a fuel which comprises a major proportion of a fuel oil which consists of

-   (A) from 0.1 to 75% by weight of at least one biofuel oil which is     based on fatty acid esters, and -   (B) from 25 to 99.9% by weight of middle distillates of fossil     origin and/or of vegetable and/or animal origin, which are     essentially hydrocarbon mixtures and are free of fatty acid esters,     and a minor proportion of specific representatives of such oligo- or     polyamines.

Since biofuel oils, which are usually also referred to as “biodiesel” and usually comprise high proportions of unsaturated fatty acid esters are, owing to their chemical structure, more sensitive to oxidative decomposition by atmospheric oxygen than fuel oils of fossil origin, their oxidation stability in the course of storage is often improved by admixing them with small amounts of antioxidants. Antioxidants typically used for this purpose are sterically hindered phenols, for example 2,6-di-tert-butyl-4-methylphenol (“BHT”), 3-tert-butylhydroxyanisole (“BHA”) and tert-butylhydroquinone (“TBHQ”), as described in the literature article by Mittelbach and Schober in JAOCS, Vol. 80, 8 (2003), p. 817-823. Particular sulfide esters, trivalent phosphorus compounds, metal dithiocarbamates and metal dithiophosphates are also used for this purpose.

It is also known from a technical information document from BASF Aktiengesellschaft from November 1997 that Kerobit® BPD, a secondary aromatic amine of the N,N′-di-sec-butyl-p-phenylenediamine structure, is suitable as an antioxidant for gasolines and jet fuels.

WO 03/095593 recommends succinimides obtainable by reacting C₄- to C₂₉-alkyl- or -alkenylsuccinic anhydride and primary amines, which may also be polyamines, as additives for improving the thermal stability of aviation fuels, and also of biofuels such as vegetable oils and esters of vegetable oils.

The documents WO 02/102942 and EP-A 1 568 756 disclose the suitability of polyisobutenylsuccinimides as detergents in biofuel oils.

WO 94/19430 describes reaction products of polyamines with acylating agents, especially with fatty acids, as antifoams for mixtures of biofuel oil and middle distillates of fossil origin.

DE 10 2004 024 532 A1 discloses fuel oils which comprise a major proportion of a mixture of middle distillate fuel oil and biofuel oil, and a minor proportion of oil-soluble oligo- or polyethyleneimines which have been alkoxylated with ethylene oxide or propylene oxide and thus have ether oxygen functions. These alkoxylated oligo- or polyethyl-eneimines, however, act there as demulsifiers for mixtures of fuel and water.

The stabilizing action of the abovementioned antioxidants, like that of the sterically hindered phenols, in the biofuel oil is, however, generally still insufficient. In particular, the dosages needed are too high. It was therefore an object of the invention to provide more effective antioxidants for biofuel oils.

The object is achieved by the use of oligo- or polyamines which have a number-average molecular weight of from 46 to 70 000 and are free of phenolic hydroxyl groups for increasing the oxidation stability.

Cationic structures, especially ammonium or substituted ammonium compounds, are preferably excluded in the oligo- or polyamines to be used in accordance with the invention.

The number-average molecular weight of the oligo- or polyamines is from 46 (for diaminomethane as the smallest possible representative) to 70 000, although this upper limit is not critical. Preferred ranges for the number-average molecular weight are from 58 (for example for 1,2-ethylenediamine) to 40 000, especially from 116 to 10 000, in particular from 130 to 5000, most preferably from 200 to 2000.

The oligo- or polyamines mentioned have preferably from 2 to 10, especially from 2 to 6, in particular from 2 to 5 nitrogen atoms in the molecule.

In a preferred embodiment, the present invention relates to the inventive use of oligo- or polyamines of the general formula I

in which

the R¹ to R⁶ radicals are each independently hydrogen, C₁- to C₃₀-alkyl groups, C₅- to C₈-cycloalkyl groups, C₁- to C₂₉-alkylcarbonyl groups or C₂- to C₈-cyanoalkyl groups, where the R¹ and R² and/or R⁵ and R⁶ radicals may in each case also, together with the nitrogen atom which bears them, form a five- or six-membered, saturated or unsaturated ring which may also have further heteroatoms (typically nitrogen and/or oxygen and/or sulfur) and/or carbonyl carbon atoms and bear additional substituents, or in each case also together be a methylidene moiety which may be substituted by C1- to C₃₀-alkyl groups and/or C₆- to C₁₂-aryl groups,

the bridging members A¹ to A³ are each independently C₁- to C₁₂-alkylene groups and/or C₆- to C₁₂-arylene groups, where the R¹ and/or R⁵ radicals may in each case also, together with the nitrogen atom which bears them and a carbon atom of an alkylene group A¹ or A³, form a five- or six-membered, saturated or unsaturated ring which may also have further heteroatoms (typically nitrogen and/or oxygen and/or sulfur and/or carbonyl carbon atoms and bear additional substituents, and

the variables n and m are each integers from 0 to 30.

The optional substitutents mentioned on the five- or six-membered, saturated or unsaturated rings may, for example, be relatively long-chain or relatively short-chain hydrocarbyl radicals, especially appropriate linear or branched alkyl radicals. Useful radicals here are firstly hydrocarbyl radicals having from 1 to 30, preferably from 1 to 20 carbon atoms, and secondly long-chain hydrocarbyl radicals having from 30 to 200, preferably from 40 to 100 carbon atoms, for example appropriate polyisobutenyl radicals.

The oligo- or polyamines to be used in the context of the present invention comprise preferably, as R¹ to R⁶ radicals, at least one hydrocarbyl radical having from 1 to 30 carbon atoms, especially having from 2 to 30 carbon atoms, more preferably having from 3 to 30 carbon atoms, in particular having from 5 to 30 carbon atoms. Such relatively long-chain hydrocarbyl radicals ensure sufficient fuel solubility. In a particularly preferred embodiment, the present invention therefore relates to a fuel which comprises a minor proportion of an oligo- or polyamine which has at least one such hydrocarbyl radical having from 5 to 30, especially from 10 to 22, in particular from 12 to 18 carbon atoms.

Examples of such relatively long-chain hydrocarbyl radicals include pure aliphatic linear or branched hydrocarbon radicals which may be of synthetic or natural origin. Examples thereof are methyl, ethyl, n-propyl, isopropyl, n-butyl, 2-butyl, isobutyl, tert-butyl, pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 2,2-dimethylpropyl, 1-ethylpropyl, n-hexyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 1-methylpentyl, 2-methylpentyl, 3-methylpentyl, 4-methylpentyl, 1,1-dimethyl butyl, 1,2-dimethylbutyl, 1,3-dimethylbutyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl, 3,3-dimethylbutyl, 1-ethylbutyl, 2-ethylbutyl, 1,1,2-trimethylpropyl, 1,2,2-trimethylpropyl, 1-ethyl-1-methylpropyl, 1-ethyl-2-methylpropyl, n-heptyl, n-octyl, n-nonyl, isononyl, 2-ethylhexyl, 2-propylheptyl, n-decyl, n-undecyl, n-dodecyl (lauryl), n-tridecyl, isotridecyl, n-tetradecyl (myristyl), n-pentadecyl, n-hexadecyl (palmityl), n-heptadecyl, n-octadecyl (stearyl), n-nonadecyl and n-eicosyl. These relatively long-chain hydrocarbyl radicals may also be of unsaturated nature, for example oleyl, linolyl or linolenyl.

Alicyclic hydrocarbyl radicals are also suitable as substituents on the aliphatic oligo- or polyamines in the context of the present invention. Examples thereof are cyclopentyl, methylcyclopentyl, cyclohexyl, methylcyclohexyl, dimethylcyclohexyl, cycloheptyl and cyclooctyl.

Suitable relatively long-chain hydrocarbyl radicals are also linear or branched alkylcarbonyl radicals, especially radicals of corresponding fatty acids having from 5 to 30, especially from 10 to 22, in particular from 12 to 18 carbon atoms. Examples thereof are formyl, acetyl, n-propanoyl, isopropanoyl, n-butanoyl, 2-butanoyl, isobutanoyl, tertbutanoyl, pentanoyl, 1-methylbutanoyl, 2-methylbutanoyl, 3-methylbutanoyl, 2,2-dimethylpropanoyl, 1-ethylpropanoyl, n-hexanoyl, 1,1-dimethylpropanoyl, 1,2-dimethylpropanoyl, 1-methylpentanoyl, 2-methylpentanoyl, 3-methylpentanoyl, 4-methylpentanoyl, 1,1-dimethylbutanoyl, 1,2-dimethylbutanoyl, 1,3-dimethylbutanoyl, 2,2-dimethylbutanoyl, 2,3-dimethylbutanoyl, 3,3-dimethylbutanoyl, 1-ethylbutanoyl, 2-ethylbutanoyl, 1,1,2-trimethylpropanoyl, 1,2,2-trimethylpropanoyl, 1-ethyl-1-methylpropanoyl, 1-ethyl-2-methylpropanoyl, n-heptanoyl, n-octanoyl, n-nonanoyl, iso-nonanoyl, 2-ethylhexanoyl, 2-propylheptanoyl, n-decanoyl, n-undecanoyl, n-dodecanoyl (lauroyl), n-tridecanoyl, isotrideanocyl, n-tetradecanoyl (myristoyl), n-pentadecanoyl, n-hexadecanoyl (palmitoyl), n-heptadecanoyl, n-octadecanoyl (stearoyl), n-nonadecanoyl and n-eicosanoyl. These relatively long-chain alkylcarbonyl radicals may also be of unsaturated nature, for example oleoyl, linoloyl or linolenoyl.

The natural raw material sources used for the abovementioned relatively long-chain aliphatic hydrocarbon radicals or relatively long-chain alkylcarbonyl radicals are in particular vegetable or animal fats or oils, for example coconut oil, tall oil, palm oil, rapeseed oil, soybean oil or jatropha oil. In the oligo- or polyamines which have relatively long-chain hydrocarbyl radicals and are based on these raw materials, the hydrocarbyl radicals are inevitably present as homologous mixtures and/or as mixtures of saturated and unsaturated chains.

Oligo- or polyamines substituted by the abovementioned alkylcarbonyl radicals can be prepared in a simple manner familiar to those skilled in the art from the amine substrate and a reactive derivative of the corresponding fatty acid, for example a lower alkyl ester, a halide such as the fatty acid chloride or the anhydride.

The abovementioned alkylcarbonyl radicals in the aliphatic oligo- or polyamines can be converted, for example by common hydrogenation processes, to the corresponding relatively long-chain aliphatic hydrocarbon radicals having the same carbon number.

Suitable bridging members A₁ to A³ are in principle all divalent linear or branched aliphatic and divalent aromatic hydrocarbon structures, but preference is given to polyalkylene groups of the formula —(CH₂)_(m)— in which m is from 1 to 12, especially from 2 to 6, in particular from 2 to 4, most preferably 2 or 3. 1,2-Ethylene and 1,3-propylene are thus particularly favorable. In the case of bridging members having 2 or 3 carbon atoms, suitable bridging members as well as the α,ω-bonding hydrocarbon structures are also nonlinear bridging members such as 1,1-ethylene, 1,1-propylene, 2,2-propylene and 1,2-propylene. Examples of divalent aromatic bridging members A¹ to A³ are ortho, meta- and para-phenylene. The bridging members A¹ to A³ may be the same or different.

The variables n and m are each independently integers from 0 to 30, but this upper limit is not critical. n and m are preferably each independently from 0 to 6, especially from 0 to 4, in particular from 0 to 2, most preferably 0 or 1.

In a particularly preferred embodiment, the present invention relates to the use of oligo-or polyamines of the general formula I in which the R¹ to R⁶ radicals are each independently hydrogen, to C₁- to C₂₂-alkyl groups, C₅- or C₆-cycloalkyl groups or C₁- to C₂₁-alkylcarbonyl groups,

the bridging members A¹ to A³ are each independently C₂- or C₃-alkylene groups and

the variables n and m are each 0 or 1,

with the proviso that at least one of the R¹ to R⁶ radicals is a C₅- to C₃₀-alkyl group, especially a C₁₀- to C₂₀-alkyl group, a C₅- or C₆-cycloalkyl group or a C₄- to C₂₉-alkylcarbonyl group, especially a C₉- to C₁₉-alkylcarbonyl group.

Typical individual examples of such oligo- or polyamines with relatively short-chain or relatively long-chain hydrocarbyl radicals which can be used for the present invention are as follows:

-   -   N,N′-dimethyl-1,3-propylenediamine     -   N,N′-diethyl-1,3-propylenediamine     -   N-isopropyl-1,3-propylenediamine     -   N,N′-di-sec-butyl-p-phenylenediamine     -   N-dodecyl-1,3-propylenediamine     -   N-dodecanoyl-1,3-propylenediamine     -   N-tetradecyl-1,3-propylenediamine     -   N-tetradecanoyl-1,3-propylenediamine     -   N-hexadecyl-1,3-propylenediamine     -   N-hexadecanoyl-1,3-propylenediamine     -   N-octadecyl-1,3-propylenediamine     -   N-octadecanoyl-1,3-propylenediamine     -   N-oleyl-1,3-propylenediamine     -   N-oleoyl-1,3-propylenediamine     -   N-cyclohexyl-1,3-propylenediamine     -   N-(3-aminopropyl)cocoamine (main component:         N-dodecyl-1,3-propylenediamine)     -   N-(3-aminopropyl)tallamine (main component:         N-oleyl-1,3-propylenediamine)     -   N-(3-aminopropyl)palmamine (main component:         N-hexadecyl-1,3-propylene-diamine)

In a further particularly preferred embodiment, the present invention relates to the use of oligo- or polyamines of the general formula I in which all R¹ to R⁶ radicals are hydrogen. These oligo- or polyamines are in particular unsubstituted oligo- and polyalkyleneamines with linear alkylene bridges. Examples thereof are:

-   -   1,2-ethylenediamine     -   1,3-propylenediamine     -   1,4-butylenediamine     -   1,6-hexylenediamine     -   1,8-octylenediamine     -   diethylenetriamine     -   dipropylenetriamine     -   N-(3-aminopropyl)-1,4-butylenediamine     -   triethylenetetramine     -   tripropylenetetramine     -   N,N′-bis(3-aminopropylene)-1,2-ethylenediamine     -   tetraethylenepentamine     -   tetrapropylenepentamine     -   pentaethylenehexamine     -   pentapropylenehexamine

Further examples of representatives of the oligo- and polyamines of the general formula I which may be used for the present invention are as follows:

-   -   N-2-cyanoethyl-N′,N′-dimethyl-1,3-propylenediamine as a         representative of a cyanoalkyl-substituted oligo- or polyamine     -   N-(3-amino-2,2-dimethylpropyl)pyrrolidine,         N-(3-aminopropyl)pyrazan, N-(3-aminopropyl)morpholine,         N-(3-aminopropyl)-N′-methylpyrazan,         N-(3-aminopropyl)-N′-(2-hydroxyethyl)pyrazan and         N-(3-aminopropyl)imidazole as representatives of oligo- or         polyamines with R¹ and R² radicals which, together with the         nitrogen atom bearing them, form a ring which may also have         further heteroatoms and may bear additional substituents     -   N-[N′-(2-aminoethyl)-2-aminoethyl)]polyisobutenylsuccinimide and         N-[N″-{N′-(2-aminoethyl)-2-aminoethyl}-2-aminoethyl]polyisobutenylsuccinimide         (having a weight-average molecular weight of the polyisobutenyl         radical of 1000 in each case), preparable by reacting the         corresponding polyisobutenylsuccinic anhydride with         diethylenetriamine or triethylenetetramine, as representatives         of oligo- or polyamines with R¹ and R² radicals which, together         with the nitrogen atom bearing them, form a ring which may also         have carbonyl carbon atoms and may bear additional substituents     -   N,N′-bis[3-(phenylazamethine)propyl]-1,2-ethylenediamine of the         formula Ph—CH=N—(CH₂)₃—NH—(CH₂)₂—NH—(CH₂)₃-N=CH—Ph as a         representative of oligo- or polyamines with R¹ and R² radicals         which are together a methylidene moiety which can be substituted         by alkyl or aryl groups     -   4-(2-aminoethyl)imidazole as a representative of oligo- or         polyamines in which the R¹ radical, together with the nitrogen         atom bearing them and a carbon atom of the alkylene group A¹,         can form a five- or six-membered ring which may also have         further heteroatoms.

Since some of the oligo- and polyamines described have not been described to date as ingredients of biofuel oils, the present invention also provides a mixture of oligo- or polyamines of the general formula I in which

the R¹ to R⁶ radicals are each independently hydrogen, C₁ - to C₃₀-alkyl groups, C₅- to C₈-cycloalkyl groups or C₂- to C₈-cyanoalkyl groups, where the R¹ and R² and/or R⁵ and R⁶ radicals may in each case also, together with the nitrogen atom which bears them, form a five- or six-membered, saturated or unsaturated ring which may also have further heteroatoms and bear additional substituents, or in each case also together be a methylidene moiety which may be substituted by C₁- to C₃₀-alkyl groups and/or C₆- to C₁₂-aryl groups,

the bridging members A¹ to A³ are each independently C₁- to C₁₂-alkylene groups and/or C₆- to C₁₂-arylene groups, where the R¹ and/or R⁵ radicals may in each case also, together with the nitrogen atom which bears them and a carbon atom of an alkylene group A¹ or A³, form a five- or six-membered, saturated or unsaturated ring which may also have further heteroatoms and/or carbonyl carbon atoms and bear additional substituents, and

the variables n and m are each integers from 0 to 30 and biofuel oils which are based on fatty acid esters in a weight ratio of from 1:100 000 to 1:100, preferably from 1:50 000 to 1:500, especially from 1:20 000 to 1:1000, in particular from 1:10 000 to 1:2000.

Since some of the oligo- and polyamines described have not been described to date as ingredients of fuel oils composed of biofuel oils and conventional middle distillates, the present invention also provides a fuel which comprises a major proportion of a fuel oil which consists of

-   (A) from 0.1 to 75% by weight of at least one biofuel oil which is     based on fatty acid esters, and -   (B) from 25 to 99.9% by weight of middle distillates of fossil     origin and/or of vegetable and/or animal origin, which are     essentially hydrocarbon mixtures and are free of fatty acid esters,

and a minor proportion of at least one oligo- or polyamine of the general formula I, in which

the R¹ to R⁶ radicals are each independently hydrogen, C₁- to C₃₀-alkyl groups, C₅- to C₈-cycloalkyl groups or C₂- to C₈-cyanoalkyl groups, where the R¹ and R² and/or R⁵ and R⁶ radicals may in each case also, together with the nitrogen atom which bears them, form a five- or six-membered, saturated or unsaturated ring which may also have further heteroatoms and bear additional substituents, or in each case also together be a methylidene moiety which may be substituted by C₁- to C₃₀-alkyl groups and/or C₆- to C₁₂-aryl groups,

the bridging members A¹ to A³ are each independently C₁- to C₁₂-alkylene groups and/or C₆- to C₁₂-arylene groups, where the R¹ and/or R⁵ radicals may in each case also, together with the nitrogen atom which bears them and a carbon atom of an alkylene group A¹ or A³, form a five- or six-membered, saturated or unsaturated ring which may also have further heteroatoms and/or carbonyl carbon atoms and bear additional substituents, and

the variables n and m are each integers from 0 to 30.

The fuel component (A) is usually also referred to as “biodiesel”. This biofuel oil (A) preferably essentially comprises alkyl esters of fatty acids which derive from vegetable and/or animal oils and/or fats. Alkyl esters are typically understood to mean lower alkyl esters, especially C₁- to C₄-alkyl esters, which are obtainable by transesterifying the glycerides, especially triglycerides, which occur in vegetable and/or animal oils and/or fats by means of lower alcohols, for example ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol or in particular methanol (“FAME”:fatty acid methyl esters).

Examples of vegetable oils which can be converted to corresponding alkyl esters and can thus serve as the basis of biodiesel are castor oil, olive oil, peanut oil, palm kernel oil, coconut oil, mustard oil, cottonseed oil and especially sunflower oil, palm oil, soybean oil and rapeseed oil. Further examples include oils which can be obtained from wheat, jute, sesame and shea tree nut; it is also possible to use arachis oil, jatropha oil and linseed oil. The extraction of these oils and their conversion to the alkyl esters are known from the prior art or can be derived therefrom.

It is also possible to convert already used vegetable oils, for example used deep fat fryer oil, if appropriate after appropriate cleaning, to alkyl esters and thus for them to serve as the basis for biodiesel.

Vegetable fats can in principle likewise be used as a source for biodiesel, but play a minor role.

Examples of animal fats and oils which are converted to corresponding alkyl esters and can thus serve as the basis of biodiesel are fish oil, bovine tallow, porcine tallow and similar fats and oils obtained as wastes in the slaughter or utilization of farm animals or wild animals.

The saturated or unsaturated fatty acids which underlie the vegetable and/or animal oils and/or fats mentioned, which usually have from 12 to 22 carbon atoms and may bear additional functional groups such as hydroxyl groups, and occur in the alkyl esters, are in particular lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linolic acid, linolenic acid, elaidic acid, erucic acid and ricinoleic acid, especially in the form of mixtures of such fatty acids.

Typical lower alkyl esters based on vegetable and/or animal oils and/or fats, which find use as biodiesel or biodiesel components, are, for example, sunflower methyl ester, palm oil methyl ester (“PME”), soybean oil methyl ester (“SME”) and in particular rapeseed oil methyl ester (“RME”).

However, it is also possible to use the monoglycerides, diglycerides and especially triglycerides themselves, for example castor oil, or mixtures of such glycerides, as biodiesel or components for biodiesel.

In the context of the present invention, the fuel component (B) shall be understood to mean middle distillate fuels boiling in the range from 120 to 450° C. Such middle distillate fuels are used in particular as diesel fuel, heating oil or kerosene, particular preference being given to diesel fuel and heating oil.

Middle distillate fuels refer to fuels which are obtained by distilling crude oil and boil within the range from 120 to 450° C. Preference is given to using low-sulfur middle distillates, i.e. those which comprise less than 350 ppm of sulfur, especially less than 200 ppm of sulfur, in particular less than 50 ppm of sulfur. In special cases, they comprise less than 10 ppm of sulfur; these middle distillates are also referred to as “sulfur-free”. They are generally crude oil distillates which have been subjected to refining under hydrogenating, conditions and which therefore comprise only small proportions of polyaromatic and polar compounds. They are preferably those middle distillates which have 95% distillation points below 370° C., in particular below 350° C. and in special cases below 330° C.

Low-sulfur and sulfur-free middle distillates may be obtained from relatively heavy crude oil fractions which cannot be distilled under atmospheric pressure. Typical conversion processes for preparing middle distillates from heavy crude oil fractions include: hydrocracking, thermal cracking, catalytic cracking, coking processes and/or visbreaking. Depending on the process, these middle distillates are obtained in low-sulfur or sulfur-free form, or are subjected to refining under hydrogenating conditions.

The middle distillates preferably have aromatics contents of below 28% by weight, especially below 20% by weight. The content of normal paraffins is between 5% by weight and 50% by weight, preferably between 10 and 35% by weight.

The middle distillates referred to as fuel component (B) shall also be understood here to mean middle distillates which can either be derived indirectly from fossil sources such as mineral oil or natural gas, or else can be prepared from biomass via gasification and subsequent hydrogenation. A typical example of a middle distillate fuel which is derived indirectly from fossil sources is the GTL (“gas-to-liquid”) diesel fuel obtained by means of Fischer-Tropsch synthesis. A middle distillate is prepared from biomass, for example via the BTL (“bio-to-liquid”) process, and can either be used alone or in a mixture with other middle distillates as fuel component (B). The middle distillates also include hydrocarbons which are obtained by the hydrogenation of fats and fatty oils. They comprise predominantly n-paraffins. It is common to the middle distillate fuels mentioned that they are essentially hydrocarbon mixtures and are free of fatty acid esters.

The qualities of the heating oils and diesel fuels are laid down in more detail, for example, in DIN 51603 and EN 590 (cf. also Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, volume A12, p. 617 ff., which is hereby incorporated explicitly by reference).

The fuel described usually also comprises the additives customary therefor, such as flow improvers for improving the cold performance, especially cold flow improvers (“middle distillate flow improvers”), nucleators, paraffin dispersants (“wax anti settling additives”) and mixtures thereof, and also conductivity improvers, corrosion protection additives, lubricity additives, antioxidants, metal deactivators, antifoams, demulsifiers, detergents, cetane number improvers, solvents or diluents, dyes or fragrances or mixtures thereof.

Especially owing to the olefinically unsaturated fatty acid units present in the biofuel oil of fuel component (A), the biofuel oil is unstable toward atmospheric oxygen and decomposes gradually by oxidation in the course of its storage in pure form and also in a mixture with middle distillates of fuel component (B) when suitable precautionary measures are not taken. The addition of customary antioxidants, such as sterically hindered phenols, for example BHT, BHA or TBHQ, has not been found to be effective enough—even in relatively high dosages. However, the above-described oligo- or polyamines fulfill this objective as improved antioxidants in a satisfactory manner. They generally also have better solubility in the biofuel oil and in the corresponding fuel comprising the biofuel oil.

A useful method for determining the oxidation stability of biofuel oils such as FAME has been found to be the so-called Rancimat method to European standard 14 112, in which an air stream is passed through the biofuel oil under controlled conditions at relatively high temperature (110° C.), and the volatile acidic addition products formed in the oxidation are collected and analyzed by conductometry. The longer the measured induction time (incubation time) until the rise in the conductivity curve, the more stable the biofuel. Desired values for the induction time are over 7 hours, especially over 8 hours, in particular over 10 hours, with a minimum dosage of the antioxidant used. Most biofuels have induction times of less than 7 hours as base values. In the case of biofuel oils and also in the case of mixtures of biofuel oils and conventional middle distillates of fossil origin, the induction times are significantly prolonged with the oligo- or polyamines described. There is interest in a minimum dosage of the antioxidant in order to save costs and in order to restrict the risk of interactions with other active ingredients in the biofuel oil or in the fuel.

The oligo- or polyamines described are dosed in the inventive fuel typically in an amount of from 10 to 10 000 ppm by weight based on the amount of the biofuel (A). Preferred dosage ranges are from 20 to 2000 ppm by weight, especially from 50 to 1000 ppm by weight, in particular from 100 to 500 ppm by weight. Most of the oligo- or polyamines described meet the requirement for a long induction time in the Ranzimate test even with a dosage of 500 ppm by weight or less.

A mixture of the oligo- or polyamines described with conventional antioxidants, especially with sterically hindered phenols, for example BHT, BHA, TBHQ, trimethylhydroquinone or bisphenol A, preferably in a weight ratio of from 10:1 to 1:10, in particular from 3:1 to 1:3, can lead to a further increase in the antioxidative and hence stabilizing action in the biofuel oil.

For the dosage of the oligo- or polyamines described to the biofuel oils or to the fuel, it has been found to be advantageous to dissolve the amines in a solvent beforehand in order that liquid dosage is possible in every case, since many of the oligo- or polyamines mentioned are solid or waxy substances. Suitable solvents in this context are in particular alcohols such as n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, 2-ethylhexanol or 2-propylheptanol, carboxylic esters or fatty acid esters such as rapeseed oil methyl ester, or amines such as dimethylamine, trimethylamine, piperidine or morpholine.

Owing to their simple mode of preparation which is familiar to those skilled in the art, the aliphatic oligo- or polyamines described can be used in sufficiently pure form, i.e. largely free of traces of metals such as iron, sodium or potassium, since especially metal traces in fuels can lead easily to faults in the engine and in any exhaust gas catalytic converter system connected downstream.

The examples which follow are intended to illustrate the present invention without restricting it.

For the oligoamines listed below as antioxidants, the induction times [in hours] in pure biofuel oil and in a mixture of a biofuel oil and a conventional middle distillate of fossil origin were determined as a function of the dosage in the Ranzimate test to European standard 14 112.

Oligoamines used:

-   -   “A1”=tetraethylenepentamine     -   “A2”=diethylenetriamine     -   “A3”=N-(3-aminopropyl)-1,4-butylenediamine     -   “A4”=N-(3-aminopropyl)-N′-methylpyrazan     -   “A5”=N-(3-aminopropyl)cocoamine     -   “A6”=N-(3-aminopropyl)palmamine     -   “A7”=N-oleyl-1,3-propylenediamine     -   “A8”=N-cyclohexyl-1,3-propylenediamine     -   “A9”=N-(3-aminopropyl)imidazole     -   “A10”=N-2-cyanoethyl-N′,N′-dimethyl-1,3-propylenediamine     -   “A11”=N,N′-bis[3-(phenylazamethin)propyl]-1,2-ethylenediamine     -   “A12”=4-(2-aminoethyl)imidazole     -   “A13”=N-(3-amino-2,2-dimethylpropyl)pyrrolidine

Prior art antioxidant used for comparison:

-   -   “BHT”=2,6-di-tert-butyl-4-methylphenol

Fuel oils used:

-   “B1”=commercial rapeseed oil methyl ester (Camp-Biodiesel,     Ochsenfurt) -   “B2”=mixture of 50% by volume of B1 and 50% by volume of commercial     diesel fuel of fossil origin

The table which follows shows the results of the determinations:

Example Biofuel- Anti- Dosage Induction No. Remark oil oxidant [ppm by wt] time [h] Base value B1 none 0 6.3 C1 comparison B1 BHT 200 7.5 1 inventive B1 A1 200 13.3 2 inventive B1 A2 200 10.7 3 inventive B1 A3 200 11.6 4 inventive B1 A4 200 10.3 C2 comparison B1 BHT 1000 7.9 5 inventive B1 A5 1000 16.7 6 inventive B1 A6 1000 11.5 7 inventive B1 A7 1000 16.8 8 inventive B1 A8 1000 10.1 9 inventive B1 A9 1000 11.9 10 inventive B1 A10 1000 11.3 11 inventive B1 A11 1000 17.4 12 inventive B1 A12 1000 18.7 13 inventive B1 A13 1000 11.4 Base value B2 none 0 10.9 V3 comparison B2 BHT 200 15.1 13 inventive B2 A1 200 >24 14 inventive B2 A2 200 18.6 15 inventive B2 A3 200 >24 16 inventive B2 A4 200 >24

The dosages are each based on the active substance; the additives were metered in as 10% by weight solutions in 2-ethylhexanol. 

1-8. (canceled)
 9. A method for inhibiting oxidation of a biofuel oil comprising (A) from 0.1 to 75% by weight of at least one biofuel oil which is based on fatty acid esters, and (B) from 25 to 99.9% by weight of middle distillates of fossil origin, of vegetable origin, of animal origin, or a combination thereof, which are essentially hydrocarbon mixtures and are free of fatty acid esters, said method, comprising: mixing the biofuel oil with at least one of an oligoamine and a polyamine that has a number-average molecular weight of from 46 to 70 000, is free of phenolic hydroxyl groups and is represented by formula I

in which each R¹ to R⁶ radical is independently a hydrogen, a C₁- to C₃₀-alkyl group, a C₅- to C₈-cycloalkyl group, a C₁- to C₂₉-alkylcarbonyl group or a C₂- to C₈-cyanoalkyl group, where the R¹ and R² radicals, the R⁵ and R⁶ radicals, or the R¹, R², R⁵ and R⁶ radicals may in each case also, together with the nitrogen atom which bears them, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and bear additional substituents, or in each case also together be a methylidene moiety which may be substituted by at least one of a C₁- to C₃₀-alkyl group and a C₆- to C₁₂-aryl group, each A¹ to A³ is independently a C₁- to C₁₂-alkylene group, where at least one of the R¹ and R⁵ radicals may in each case also, together with the nitrogen atom which bears them and a carbon atom of an alkylene group A¹ or A³, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and/or carbonyl carbon atoms and bear additional substituents, and the variables n and m are each integers from 0 to 30, said oligoamine or polyamine operable for increasing the oxidation stability of biofuel oils based on fatty acid esters, or of mixtures of such biofuel oils with middle distillates of fossil origin and/or of vegetable and/or animal origin, which are essentially hydrocarbon mixtures and are free of fatty acid esters.
 10. The method according to claim 9, which has as R¹ to R⁶ radicals, at least one hydrocarbyl radical having from 1 to 30 carbon atoms.
 11. The method according to claim 10, in which each R¹ to R⁶ radical is independently a hydrogen, a C₁- to C₂₂-alkyl group, a C₅- or C₆-cycloalkyl group, or a C₁- to C₂₁-alkylcarbonyl group, each A¹ to A³ is independently a C₂- or C₃-alkylene group, and the variables n and m are each 0 or 1, with the proviso that at least one of the R¹ to R⁶ radicals is a C₅- to C₃₀-alkyl group, a C₅- or C₆-cycloalkyl group or a C₄- to C₂₉-alkylcarbonyl group.
 12. The method according to claim 11, in which all R¹ to R⁶ radicals are hydrogen.
 13. The method according to claim 9, wherein the at least one of an oligoamine and a polyamine is present in the biofuel oil in an amount of from 10 to 10 000 ppm by weight based on the amount of the biofuel oil.
 14. A mixture comprising biofuel oils which are based on fatty acid esters in a weight ratio of from 1:100 000 to 1:100 and at least two oligoamines or polyamines represented by formula I

in which each R¹ to R⁶ radical is independently a hydrogen, a C₁- to C₃₀-alkyl group, a C₅- to C₈-cycloalkyl group, a C₁- to C₂₉-alkylcarbonyl group or a C₂- to C₈-cyanoalkyl group, where the R¹ and R² radicals, the R⁵ and R⁶ radicals, or the R¹, R², R⁵ and R⁶ radicals may in each case also, together with the nitrogen atom which bears them, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and bear additional substituents, or in each case also together be a methylidene moiety which may be substituted by at least one of a C₁- to C₃₀-alkyl group and a C₆- to C₁₂-aryl group, each A¹ to A³ is independently a C₁- to C₁₂-alkylene group, where at least one of the R¹ and R⁵ radicals may in each case also, together with the nitrogen atom which bears them and a carbon atom of an alkylene group A¹ or A³, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and/or carbonyl carbon atoms and bear additional substituents, and the variables n and m are each integers from 0 to
 30. 15. A fuel comprising a major proportion of a fuel oil which comprises: (C) from 0.1 to 75% by weight of at least one biofuel oil which is based on fatty acid esters, and (D) from 25 to 99.9% by weight of middle distillates of fossil origin, of vegetable origin, of animal origin, or a combination thereof, which are essentially hydrocarbon mixtures and are free of fatty acid esters, and a minor proportion of at least one of an oligoamine and a polyamine that has a number-average molecular weight of from 46 to 70 000, is free of phenolic hydroxyl groups and is represented by formula I

in which each R¹ to R⁶ radical is independently a hydrogen, a C₁- to C₃₀-alkyl group, a C₅- to C₈-cycloalkyl group, a C₁- to C₂₉-alkylcarbonyl group or a C₂- to C₈-cyanoalkyl group, where the R¹ and R² radicals, the R⁵ and R⁶ radicals, or the R¹, R², R⁵ and R⁶ radicals may in each case also, together with the nitrogen atom which bears them, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and bear additional substituents, or in each case also together be a methylidene moiety which may be substituted by at least one of a C₁- to C₃₀-alkyl group and a C₆- to C₁₂-aryl group, each A¹ to A³ is independently a C₁- to C₁₂-alkylene group, where at least one of the R¹ and R⁵ radicals may in each case also, together with the nitrogen atom which bears them and a carbon atom of an alkylene group A¹ or A³, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and/or carbonyl carbon atoms and bear additional substituents, and the variables n and m are each integers from 0 to
 30. 16. A fuel comprising a major proportion of a fuel oil which consists of: (E) from 0.1 to 75% by weight of at least one biofuel oil which is based on fatty acid esters, and (F) from 25 to 99.9% by weight of middle distillates of fossil origin, of vegetable origin, of animal origin, or a combination thereof, which are essentially hydrocarbon mixtures and are free of fatty acid esters, and a minor proportion of at least one of an oligoamine and a polyamine that has a number-average molecular weight of from 46 to 70 000, is free of phenolic hydroxyl groups and is represented by formula I

in which each R¹ to R⁶ radical is independently a hydrogen, a C₁- to C₃₀-alkyl group, a C₅- to C₈-cycloalkyl group, a C₁- to C₂₉-alkylcarbonyl group or a C₂- to C₈-cyanoalkyl group, where the R¹ and R² radicals, the R⁵ and R⁶ radicals, or the R¹, R², R⁵ and R⁶ radicals may in each case also, together with the nitrogen atom which bears them, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and bear additional substituents, or in each case also together be a methylidene moiety which may be substituted by at least one of a C₁- to C₃₀-alkyl group and a C₆- to C₁₂-aryl group, each A¹ to A³ is independently a C₁- to C₁₂-alkylene group, where at least one of the R¹ and R⁵ radicals may in each case also, together with the nitrogen atom which bears them and a carbon atom of an alkylene group A¹ or A³, form a five- or six-membered, saturated or unsaturated ring which may also have further hetero atoms and/or carbonyl carbon atoms and bear additional substituents, and the variables n and m are each integers from 0 to
 30. 17. The method according to claim 9, further comprising passing an air stream through the biofuel oil; and measuring an induction time to determine the stability of the biofuel oil.
 18. The method according to claim 17, wherein the induction time is greater than 7 hours.
 19. The method according to claim 17, wherein the induction time is greater than 8 hours.
 20. The method according to claim 17, wherein the induction time is greater than 10 hours.
 21. The method according to claim 17, wherein the induction time is from 10.1 to 18.7 hours.
 22. The method according to claim 17, wherein the induction time is greater than 24 hours. 